A fundamental problem in synthetic aperture radar technique (SAR) is that, in its traditional form it, presupposes that an aircraft moves a relatively long distance along a straight path. When the SAR technique is driven to more extreme performances, this becomes a problem also in so far as it is not even possible to fly with the accuracy that is required. Moreover, military use may make it necessary to be able to make the SAR registrations while the aircraft is maneuvering.
In the wide band low frequency SAR called CARABAS, that has been patented under the Swedish patent no. 8406007-8, motion compensation is an important problem, as a registration is carried out during an approximately 10 km long distance, during which the accelerations must be less than 10.sup.-1 -10.sup.-3 ms.sup.-2 (depending upon reconnaissance distance and desired resolution), if a traditional SAR signal processing shall be used.
Methods have evolved in the SAR technique to compensate for movements. These are essential for the SAR technique as it is regularly used today in modern reconnaissance and attack aircrafts. These methods are, however, approximative and while they work for narrow band microwave SAR, having relatively short aperture distances, they can't be used for CARABAS.
The foregoing problem is solved by the present invention.